In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is combusted, and the resulting hot combustion gases are passed through a turbine mounted on the same shaft. The flow of gas turns the turbine by contacting an airfoil portion of the turbine blade, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward. There may additionally be a bypass fan that forces air around the center core of the engine, driven by a shaft extending from the turbine section.
The turbine section of the engine is heated to high temperatures by the hot combustion gases, which are highly oxidizing and also highly corrosive. A number of the components of the turbine section, such as turbine blades and turbine vanes, are made of nickel-base alloys having aluminum present to contribute to the strengthening mechanism. The aluminum and other elements present also impart some oxidation and corrosion resistance to the material. Experience has shown, however, that as the combustion-gas temperature has increased for improved thermodynamic efficiency of the gas turbine engine, the surfaces of the base alloys are not sufficiently oxidation resistant and corrosion resistant.
Coatings have been developed to protect the surfaces of the nickel-aluminum-containing components against oxidation and corrosion more effectively than the bare base metal can protect itself. A protective coating typically includes an aluminum-enriched layer having a higher percentage of aluminum than present in the base-metal alloy. The aluminum in this aluminum-enriched layer oxidizes to form a protective aluminum oxide (alumina) scale at the surface of the aluminum-enriched layer, which scale serves as a diffusion barrier to inhibit further oxidation of the coating and thence of the underlying substrate. A ceramic may overlie the aluminum-enriched layer.
The use of aluminum-base quasicrystalline alloys, which have low thermal conductivities, has been proposed for protective coatings. These quasicrystalline alloys may contain porosity or be mixed with small amounts of heat conductive materials such as particles of aluminum. While this approach potentially has merit, it has not been optimized to reflect the realities of the practical limiting considerations for such protective coatings when used to protect nickel-base alloys in the hot-combustion-gas environment.
There is a need for an approach that makes use of the beneficial thermal properties of quasicrystalline alloys, while at the same time achieves acceptable performance in the gas turbine operating environment. The present invention fulfills this need, and further provides related advantages.